Greetings Robbie and MMD.  You asked "why can't the vertical piano action
repeat as well as a grand action?"

The reason is that grand pianos have repetition levers, and verticals
don't.  The repetition lever allows the note to be repeated when the key
has only come back half way.  On an upright the key must come back all
the way.  (That's the theory, and I have seen some uprights repeat before
the key comes all the way up, but that is the exception rather than the
rule.)

The only way to further understand this is to look at a drawing of the
action, to familiarize yourself with the parts and the concepts, then
watch the actual mechanism work.  There are excellent diagrams in the
Arthur Reblitz book, "Piano Service, Tuning and Rebuilding"

The "grand repetition action" or "double escapement action" was patented
in 1821 by Sebastion Erard (Dolge, "Pianos and their Makers").  With
some minor refinements this is the action of the grand piano today.

In regards to the Disklavier on Yamaha uprights, the regulation should
be to factory specs.  The lost motion should be checked after the player
unit has been played a while to make sure there is no "negative lost
motion" (for lack of a better term), i.e., the solenoid rods at rest
should not be pushing up on the keys even a hair.  (I am assuming the
Disklavier pushes up on the backs of the keys a la the old Marantz
Pianocorder.)  If one runs a thumb down the keys at the capstans,
glissando style, and sees some hammers wink or nod, this is an example
of "negative lost motion".

There can be no sluggishness  in the action.  If the soft pedal is
depressed then released all of the hammers should come back at the same
time.  If there are slow hammers, you will see them lagging behind.  Lost
motion and slow hammers are something the owner can and should check for
if so inclined.  To check for lost motion, (in uprights only) the hammer
should move when the key is ever so slightly depressed.

To get an idea of extreme lost motion, depress the soft pedal, then
depress a key and see how far it goes down before the hammer moves.
That's why the upright feels different in playing when the soft pedal is
used.  If the owner can detect lost motion (without the soft pedal
depressed of course) there is way too much, or the solenoid operating
the soft pedal is out of adjustment.

On the player unit, the solenoids themselves have to be repeating and not
binding.  Also they should not push up too far on the keys in the on or
play position.  Does the Disklavier have a stop rail on top of the keys
towards the rear?

Richard Moody

 [ Thanks for the tip, Richard --  Art Reblitz's description is very
 [ good, and I understand the problem now.  -- Robbie